CV Joint Failure ?

[RTH]

I've got a "bump in the night" I need help with ( or more correctly a clunk). I'm thinking it's a CV joint failure, but some reasurance would be helpful before I lift and seperate. It started with a slight clicking sound under load and progressed to a loud clunk that seems to match the wheel rotation speed, although depressing the cluch makes it go away. I've searched the threads and read the great post by Grady concerning the CV rebuild. Any thoughts about my assumption ? I've got a 79 911sc that I can't get enough of...

[ChrisBennet]

Have you checked the CV bolts? It's not uncommon for these to get loose.
-Chris

[Grady Clay]

I will second Chris’ admonition. The noise JUST BEFOR the CV comes loose can sound like a failed (or failing CV). Check all the CV bolts – inner and outer (if any). You should do this before the next drive. CVs coming loose seems to be an all-to-common preventable problem.


The two most common failure modes for CVs are having been run without proper lube (leak from boot) and cracked cage from collapsing the axial free-play. Yes, some get worn out but even that usually is due to lack of maintenance rebuilds and fresh grease. With proper maintenance, the big 108 mm CVs should conceivably outlast the car.

Best,
Grady

[bagpiper46]

This sounds like my problem. I just wish the garage was heated! it is 25 degrees out there right now. Well at least its warmer than outside!! I will jack it up tonite and have a look.
How hard is it to replace the cv joint? can a local non porsche wrench handle it (with Wayne's book and a heated shop with lift)?
I dont know if I want to try a 3 wrench project in December.

[bagpiper46]

Seems like all the bolts are in place. Hear a grinding noise on the drivers side inner joint (center of car) when I rotate the wheel in nuetral. I am 125 miles to the nearest Porsche wrench. Can a local transmission guy handle this? would it be wise to get 2 whole axles for $265 each and take it to the wrench or rent a flatbed and take it to the Porsche wrench in Denver? How much am I going to pay for the Denver Porsche wrench to do this job?
Will my local shop have the talent to do this? Thats 4 questions which should be enough. Wait! If I get brave and tackle the job myself are the joints already lubed and ready to go? Would it just be a bolts off bolts in?

HELP!!

[randywebb]

"collapsing the axial free-play"

- Grady - what is likely to cause this?

[RTH]

Thanks for the advise... I did some investigation and found that indeed the joint had failed, broken boots and dry joints. I decided to order a new axle complete.... Took me an hour to unbolt the old one and slip the new one in.... The bolts are in good condition. so I did not replace them. As I set the new axle I found the bolts I had were too long.... After further examination I found that the old and new axles look the same except for the flange on the boot which is much thicker on the old joint. Anyone know if I have the wrong axle or if I just need to get a shorter bolt ?

P.S. Bagpiper46 if you pick this up... I'd go for the change yourself.. Just pull the vent off the dryer and let it blow in the garage for a while... that should bring the temp up to just above freezing...

[rc1]

You didnt need to buy a new axel, unless the old axel was damaged in some way. All you have to do is remove the large snap-ring on the end of the axel and the old cv will slide right off. By the way, cv boots for the early VW bus are exactly the same, and cost only $7.00 each vs the porsche cv boot at $45.00. I even think the VW part is better as the rubber is pressed into yhe metal flange vs the porsche boot being attached by a large clamp. Be sure to pack it full of fresh grease. Hope this helps.

[Grady Clay]

Randy,

If you grab an axle shaft in the middle it has axial free play (there shouldn’t be any radial play.) It is worthwhile to check this periodically while turning the wheel to get the feel of good CV joints. It will have most free-play when the suspension is near normal height (not hanging.)

An advantage of doing this is you move around grease that has just been sitting there in normal operation. That tends to get more grease where the balls normally operate.

The reason for the free-play is in the Lobro constant velocity joint design. It is also necessary to accommodate lateral movement of the transmission, different alignment settings and lateral deformation of the suspension under load.

If under some unfavorable circumstance the axial free-play goes to zero, the CV balls contact the ends of the cage slots and crack the cage. Worst case the cage brakes and the CV spits out the balls.

Some model CVs have greater range than others. The big 108 mm CVs used in ’69-’71 and the Turbos have the most. With the 100 mm CVs in order to have sufficient axial play necessitated smaller balls. This reduced the load ability.

Earlier this week I measured the distance between the axle flanges on two different transmission types. The 1979 915/61 measured 247.5 mm compressed and the ’86 915/72 measured 249.5 mm compressed – a full two mm difference. That either extends each CV by 0.5 mm or compresses them by 0.5 mm depending on which way a conversion is done. Probably not enough to worry about unless it is already close.

Next week I’ll repeat my measurements and measure them extended.

All these subtle differences may not seem like much but the devil is in the details.

Best,
Grady

[randywebb]

Thanks Grady - I'm familiar with the axial free play & the design of the Lobro....

My question is what would cause the axial play to collapse....?

[rcecale]

Grady,

I've sent you an e-mail. You were asking in the "Reconstructing CV Joints" thread if I still had my old axles, and if I wouldn't mind sacrificing them for the sake of educating us Pelicans.

"Yes" to both questions. They're yours! How do I get them to you?

Randy

[Grady Clay]

Aah, dumb ‘ol me.

Usually it is from sliding into something. However, I have found cracked cages even when there wasn’t any suspension damage. I interpret that to be the end-play wasn’t sufficient for the deformation of the suspension going one direction and the transmission the other.

Those are wonderfully ingenious CV joints but don’t take well to over stressing the cage at the axial extremes. Since the CV is always running at an angle, the contact only happens one ball at a time as the CV rotates.

Thanks.

Best,
Grady

[Nine9six]

Gawd, I love Grady's posts!
When I get bored of the same ole posts, I like to search on his name just to wallow in his wisdom. I cant tell how many time the light has come on after reading one of his posts. I think its his attention to detail and ability to speak in lay man's terms.
Thanks again GC

[randywebb]

Thanks - that all makes sense. Guess they should have designed a little deeper to run them straight and not angled when they made the LWB cars....

[Grady Clay]

Randy,

They did run straight with SWB cars. That is why they could use
such short axles. With the LWB they kept everything in place
except moved the rear wheels back. In order to keep the angle
as low as possible they dramatically lengthened the axle to get
the CVs far apart.



This SWB axle is ‘late ’66-’68 Lobro and the LWB is a 100 mm
friction welded CV from a 911SC. Axles complements of
Eurosport, Ltd.

This, in turn, necessitated narrowing the axle flanges at the
transmission and changing from the well supported SWB
hub-bearing arrangement to the LWB relatively thin 4-point
bearing at the outboard end.

Here is the SWB cross-section. The distance between the
ball bearing and the roller bearing is about 11 cm.
"
"
© 1965 Dr. Ing. h.c. F. Porsche A.G.

Note the universal joint of the Nadella axle.


Here is the LWB cross-section. The distance between the
two rows of ball bearings is only about 2 cm.
“
"
© 1969 Dr. Ing. h.c. F. Porsche A.G.

You almost never see a SWB rear wheel bearing failure other
than lack of grease. On the other hand the ’69-’73 rear wheel
bearing needs to be replaced most often of any. This was the
reason for the change from the 75 mm bearing to the larger
80 mm bearing in ’74 (along with the alu trailing arm.)

It was the ’75 930 Turbo that got the real strength back with
widely spaced dual tapered roller bearings. Porsche could fit
them because of the dramatically wider rear track and thereby
keep the long axle. Another advantage of the Turbo and Turbo
Look 911s.

I think one of the fun technical aspects of the 911 is to watch
the progression as Porsche engineers learned. Of course at
the same time the car became heavier, more powerful and
had better grip through tires and suspension design. Some
times they were ahead of the curve and some times behind.

Best,
Grady

[ChrisBennet]

Grady,
I thought the CV's with the friction welded stub axle with it's 10mm bolts didn't come didn't come along until mid '85.
-Chris